Downhole fluid sampling and analysis is an important and efficient investigative technique typically used to ascertain characteristics and nature of geological formations having hydrocarbon deposits. In this, typical oilfield exploration and development includes downhole fluid sampling and analysis for determining petrophysical, mineralogical, and fluid properties of hydrocarbon reservoirs. Fluid characterization is integral to an accurate evaluation of the economic viability of a hydrocarbon reservoir formation.
Typically, a complex mixture of fluids, such as oil, gas, and water, is found downhole in reservoir formations. The downhole fluids, which are also referred to as formation fluids, have characteristics, including pressure, temperature, volume, among other fluid properties, that determine phase behavior of the various constituent elements of the fluids. In order to evaluate underground formations surrounding a borehole, it is often desirable to obtain samples of formation fluids in the borehole for purposes of characterizing the fluids, including composition analysis, fluid properties and phase behavior. Wireline formation testing tools are disclosed, for example, in U.S. Pat. Nos. 3,780,575 and 3,859,851, and the Reservoir Formation Tester (RFT) and Modular Formation Dynamics Tester (MDT) of Schlumberger are examples of sampling tools for extracting samples of formation fluids from a borehole for surface analysis.
Formation fluids under downhole conditions of composition, pressure and temperature typically are different from the fluids at surface conditions. For example, downhole temperatures in a well could range from 300 degrees F. When samples of downhole fluids are transported to the surface, change in temperature of the fluids tends to occur, with attendant changes in volume and pressure. The changes in the fluids as a result of transportation to the surface cause phase separation between gaseous and liquid phases in the samples, and changes in compositional characteristics of the formation fluids.
Techniques also are known to maintain pressure and temperature of samples extracted from a well so as to obtain samples at the surface that are representative of downhole formation fluids. In conventional systems, samples taken downhole are stored in a special chamber of the formation tester tool, and the samples are transported to the surface for laboratory analysis. During sample transfer from below surface to a surface laboratory, samples often are conveyed from one sample bottle or container to another bottle or container, such as a transportation tank. In this, samples may be damaged during the transfer from one vessel to another.
Furthermore, sample pressure and temperature frequently change during conveyance of the samples from a wellsite to a remote laboratory despite the techniques used for maintaining the samples at downhole conditions. The sample transfer and transportation procedures currently in use are known to damage or spoil formation fluid samples by bubble formation, solid precipitation in the sample, among other difficulties associated with the handling of formation fluids for surface analysis of downhole fluid characteristics.
In addition, laboratory analysis at a remote site is time consuming. Delivery of sample analysis data takes anywhere from a couple of weeks to months for a comprehensive sample analysis. This hinders the ability to satisfy users' demand for real-time results and answers (i.e., answer products). Typically, the time frame for answer products relating to surface analysis of formation fluids is a few months after a sample has been sent to a remote laboratory.
As a consequence of the shortcomings in surface analysis of formation fluids, recent developments in downhole fluid sampling and analysis include techniques for isolating and characterizing formation fluids downhole in a wellbore or borehole. In this, the MDT may include one or more fluid analysis modules, such as the Composition Fluid Analyzer (CFA) and Live Fluid Analyzer (LFA) of Schlumberger, for example, to analyze downhole fluids sampled by the tool while the fluids are still located downhole.
In downhole fluid sampling and analysis modules of the type described above, formation fluids that are to be sampled and analyzed downhole flow past a sensor module associated with the fluid sampling and analysis module, such as a spectrometer module, which analyzes the flowing fluids by infrared absorption spectroscopy, for example. In this, an Optical Fluid Analyzer (OFA), which may be located in the fluid analysis module, may identify fluids in the flow stream and quantify the oil and water content. U.S. Pat. No. 4,994,671 (incorporated herein by reference in its entirety) describes a borehole apparatus having a testing chamber, a light source, a spectral detector, a database, and a processor. Fluids drawn from the formation into the testing chamber are analyzed by directing the light at the fluids, detecting the spectrum of the transmitted and/or backscattered light, and processing the information (based on information in the database relating to different spectra), in order to characterize the formation fluids.
In addition, U.S. Pat. Nos. 5,167,149 and 5,201,220 (both incorporated herein by reference in their entirety) describe apparatus for estimating the quantity of gas present in a fluid stream. A prism is attached to a window in the fluid stream and light is directed through the prism to the window. Light reflected from the window/fluid flow interface at certain specific angles is detected and analyzed to indicate the presence of gas in the fluid flow.
As set forth in U.S. Pat. No. 5,266,800 (incorporated herein by reference in its entirety), monitoring optical absorption spectrum of fluid samples obtained over time may allow one to determine when formation fluids, rather than mud filtrates, are flowing into the fluid analysis module. Further, as described in U.S. Pat. No. 5,331,156 (incorporated herein by reference in its entirety), by making optical density (OD) measurements of the fluid stream at certain predetermined energies, oil and water fractions of a two-phase fluid stream may be quantified.
Conventionally, multiple valves are utilized in downhole fluid sampling and analysis modules of the type described above to control flow of formation fluids through the flowlines of the fluid analysis modules. For example, co-pending and commonly owned U.S. patent application Ser. No. 11/203,932, filed Aug. 15, 2005, entitled “Methods and Apparatus of Downhole Fluid Analysis”, discloses the use of a plurality of valves for isolating formation fluids in a part of the flowline of a downhole sampling and analysis module. FIG. 7 schematically represents one example of a fluid sampling and analysis module with a flowline and multiple valve configuration for downhole characterization of fluids by isolating or capturing the formation fluids. In systems of the type depicted in FIG. 7, motors are provided downhole to actuate the valves, and a driver board is configured to control operation of the valves and associated motors. Typically, seal valves are employed for purposes of opening or closing the flowlines. The seal valves also may be used for directing fluids through the fluid sampling and analysis module.
The fluid control systems of the type described above have multiple components and operating parts, and require space in the downhole modules. In consequence, there is a need for a simple, yet reliable, fluid control system that provides the functionality described above, yet requires minimal space and downhole hardware for its operations.